1. Field of the Invention:
The invention relates to a process for permanently bonding thermoplastic molding materials based on polyphenylene ethers and peroxide vulcanized EP(D)M rubbers, as well as the objects obtained by this process.
2. Discussion of the Background:
Frequently a single material cannot contribute all of the properties that are demanded of an object. Such incompatible combinations of properties are e.g. simultaneously high stability and rubber elasticity or high hardness and rigidity on the one hand and skid resistance on the other hand.
In order to provide components with properties which one single material cannot contribute, they are composed of components of different materials. Frequently, a permanent adhesion between the components of different materials is an essential pre-requisite for the functionability of such objects.
The best process for producing such bonding materials is to covulcanize the rigid molding material and the rubber material.
Thus it is known that SBR and EPR rubbers and polybutadienes have an unexpectedly high adhesion to specific plastics following longer thermal treatment. These plastics, which are characterized by the recurrent structural unit, ##STR1## comprise poly(2,6-dimethyl-1,4-phenylene) ethers, polysulfones, and polycarbonates. With polystyrenes, which do not have this structural unit, the strength of adhesion is more than 6-times smaller (P. Dreyfuss, M. L. Runge, J. Appl. Pol. Sci. 23, pp. 1863 to 1866). The authors assert that in many cases these methods yield an excellent bond between the elastomer and plastic layer, even if or precisely when the elastomers, as in the case of EPR rubber, do not contain double bonds.
In fact, there are a number of disadvantages associated with this method. The pure polyphenylene ethers play an economically subordinate role due to their poor processability and their inadequate notch impact resistance. All PPE-containing polymer mixtures contain additional polymers whose bonding properties are definitely worse. For the economically interesting polymer mixtures whose proportion of styrene is more than 10%, the method described above should, therefore, be unsuitable.
For industrial application purposes, filler-containing rubbers are generally used. The adhesion properties of the filler-containing rubbers should be worse in comparison with the studied filler-free systems.
Furthermore, a significant disadvantage is the condition that the treatment times are quite long. Multiple hours of exposure time are incompatible with modern economical production of molded articles.
Under these conditions, it is not surprising that this process has not been further developed nor has it been implemented for industrial use. If despite these objections, the process is closely scrutinized, it is clear that the authors' primary objective was to produce a physical bond between specific thermoplastics and synthetic rubbers. In particular, a cold flow was to be prevented. A chemical bond was to be avoided; nevertheless, it was found that the strength of adhesion decreases with the increased degree of cross-linking of rubber (see A. Ahagon, A. N. Gent, J. Polym. Sci.; Polym. Phys. Ed. 13, p. 1285 (1975) Summary).
If one pursues the investigations, it is clear that the adhesion values are in no way as good as shown in the article cited (see comparison tests in Table 2 of DE-OS 36 02 705). In individual cases the strength of separation is immeasurable, since the rubber has become crumbly in the course of thermal treatment.
Recently a very interesting process has been developed for producing a chemical bond between molding materials based on polyphenylene ethers (PPE) and rubbers containing specific double bonds and vulcanizable with sulfur (see DE-OS 36 02 705). In this process SBR, BR, IR, and IIR rubbers and their grafts are added. Finally it is also possible to use rubber mixtures, which can also contain CIIR, CR, and NBR rubbers in specific ratios.
The adhesions values that are sought are noticeable; however, the stability of the added rubber materials with respect to solvents leaves as much to be desired as their stability with respect to weathering.
In principle, with this process one could conceive of adding other double bond-containing rubbers, which in the conventional manner are more suitable to meeting these requirements. However, it has been demonstrated that for example EPDM rubbers have inadequate adhesion values under those process conditions that have been recognized as important. Therefore, with known processes, it is not possible to produce a bond between polyphenylene ethers and rubbers hand that combine the adequate adhesion properties with good stability with respect to solvents and weathering.